FY 2023

Simulated impact of ocean alkalinity enhancement on atmospheric CO2 removal in the Bering Sea

Wang, H., D. Pilcher, K.A. Kearney, J.N. Cross, M. Shugart, M.D. Eisaman, and B.R. Carter

Earth's Future, 11(1), doi: 10.1029/2022EF002816, View open access article online at AGU/Wiley (external link) (2023)

Ocean alkalinity enhancement (OAE) has the potential to mitigate ocean acidification (OA) and induce atmospheric carbon dioxide (CO₂) removal (CDR). We evaluate the CDR and OA mitigation impacts of a sustained point-source OAE of 1.67 × 10¹⁰ mol total alkalinity (TA) yr⁻¹ (equivalent to 667,950 metric tons NaOH yr⁻¹) in Unimak Pass, Alaska. We find the alkalinity elevation initially mitigates OA by decreasing pCO₂ and increasing aragonite saturation state and pH. Then, enhanced air-to-sea CO₂ exchange follows with an approximate e-folding time scale of 5 weeks. Meaningful modeled OA mitigation with reductions of >10 μatm pCO₂ (or just under 0.02 pH units) extends 100–100,000 km² around the TA addition site. The CDR efficiency (i.e., the experimental seawater dissolved inorganic carbon (DIC) increase divided by the maximum DIC increase expected from the added TA) after the first 3 years is 0.96 ± 0.01, reflecting essentially complete air-sea CO₂ adjustment to the additional TA. This high efficiency is potentially a unique feature of the Bering Sea related to the shallow depths and mixed layer depths. The ratio of DIC increase to the TA added is also high (≥0.85) due to the high dissolved carbon content of seawater in the Bering Sea. The air-sea gas exchange adjustment requires 3.6 months to become (>95%) complete, so the signal in dissolved carbon concentrations will likely be undetectable amid natural variability after dilution by ocean mixing. We therefore argue that modeling, on a range of scales, will need to play a major role in assessing the impacts of OAE interventions.

Plain Language Summary. The Intergovernmental Panel on Climate Change suggests that carbon dioxide (CO₂) removal (CDR) approaches will be required to stabilize the global temperature increase at 1.5–2°C. In this study, we simulated the climate mitigation impacts of adding alkalinity (equivalent to 667,950 metric ton NaOH yr⁻¹) in Unimak Pass on the southern boundary of the Bering Sea. We found that adding alkalinity can accelerate the ocean CO₂ uptake and storage and mitigate ocean acidification near the alkalinity addition. It takes about 3.6 months for the Ocean alkalinity enhancement impacted area to take up the extra CO₂. The naturally cold and carbon rich water in the Bering Sea and the tendency of Bering Sea surface waters to linger near the ocean surface without mixing into the subsurface ocean both lead to high CDR efficiencies (>96%) from alkalinity additions in the Bering Sea. However, even with high efficiency, it would take >8,000 alkalinity additions of the kind we simulated to be operating by the year 2100 to meet the target to stabilize global temperatures within the targeted range.